The Evolution of European Solar-to-X Research: From Fundamental Material Science to the SPECTRUM Polygeneration Framework

The European Union’s commitment to achieving climate neutrality by 2050 under the Green Deal and REPowerEU mandates rapid industrial decarbonization. However, the industrial sector remains heavily constrained by its reliance on fossil fuels primarily for process heat, high land-use competition, and growing environmental liabilities such as energy-intensive wastewater management. Traditional, single-product solar deployments (e.g., standalone photovoltaic or hydrogen facilities) fail to address this multi-vector energy-water nexus and suffer from poor economic viability. This paper presents a systematic review of European Union-funded Solar-to-X (S2X) research across the FP7, Horizon 2020, and Horizon Europe framework programs. Employing a thematic evolutionary framework, the study evaluates project trajectories from early-stage material science breakthroughs (NanoPEC, ArtipHyction, PECDEMO) to multi-megawatt industrial scale-ups (HYDROSOL series, PECSYS) and the emerging polygeneration paradigm (PH2OTOGEN, GH2, SOL2HY2, DESIRED, Sun-To-X, SOLEAS). By synthesising performance metrics, thermal dynamics, and co-product economics across these initiatives, this paper maps how historical lessons converge into the novel SPECTRUM framework. SPECTRUM introduces a decentralised concentrating collector that utilises physical spectral splitting to thermally decouple photovoltaic cells from absorbers, matching specific wavebands to high-efficiency processes: ultraviolet for simultaneous photocatalytic H2 evolution and industrial wastewater remediation, visible light for clean electricity, and infrared for high-temperature heat generation. Ultimately, this review establishes full-spectrum harvesting and circular polygeneration as the definitive blueprint for scalable, high-powerdensity industrial infrastructure.